The dimensionless humidity ratio is typically expressed as grams of water per kilogram of dry air, or grains of water per pound of air (7000 grains equal 1 pound). For a given DBT there will be a particular humidity ratio for which the air sample is at 100% relative humidity: the relationship reflects the physics of water and air and must be determined by measurement. It is typically plotted as the ordinate (vertical axis) of the graph. It is also known as the moisture content or mixing ratio. Humidity ratiois the proportion of mass of water vapor per unit mass of dry air at the given conditions (DBT, WBT, DPT, RH, etc.). The concept that air “holds” moisture, or that moisture “dissolves” in dry air and saturates the solution at some proportion, is erroneous (albeit widespread) see relative humidity for further details. Lines of constant RH reflect the physics of air and water: they are determined via experimental measurement. RH is dimensionless, and is usually expressed as a percentage. Relative humidity (RH) is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. The dew point temperature is measured easily and provides useful information, but is normally not considered an independent property of the air sample as it duplicates information available via other humidity properties and the saturation curve. The slope of the line of constant WBT reflects the heat of vaporization of the water required to saturate the air of a given relative humidity.ĭew point temperature (DPT) is the temperature at which a moist air sample at the same pressure would reach water vapor “saturation.” At this point further removal of heat would result in water vapor condensing into liquid water fog or, if below freezing point, solid hoarfrost. When the air sample is pre-saturated with water, the WBT will read the same as the DBT. In practice this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air (see Hygrometer).
Wet-bulb temperature (WBT)is that of an air sample after it has passed through a constant-pressure, ideal, adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. The SI units for temperature are kelvins or degrees Celsius other units are degrees Fahrenheit and degrees Rankine.
It is typically plotted as the abscissa (horizontal axis) of the graph. Dry-bulb temperature (DBT)is that of an air sample, as determined by an ordinary thermometer.